Check that both spark plugs are firing. If one isn't it doesn't have enough power to start the PTO.
My 2135 is a single cylinder Kohler. Is yours a twin?
As mentioned, disconnect the drive belt to the mower deck and make sure the PTO clutch will engage and is working unloaded.
Next check belt routing and that both blades turn freely.
Full disclosure, I bought my 2135 used, because the original purchaser could not keep it running and had given up after spending hundreds of dollars taking it back to the dealer for new switches and troubleshooting labor. For a while, I lived with it because I could trace and repair the wiring problems myself.
We have an acre of lawn and the 2135 worked well when it worked, but every time I was gone, my wife or daughter wouldn't be able to get the mowing done. Finally, they said it had to be dependable or be gone. I had to accept that the expensive factory replacement switches and wiring would always be a problem.
The soulution was to rewire the entire machine. On my 2135, the PTO switch was a continual source of trouble. The switch just is not sufficiently heavy duty nor weatherproof. The many interlocks which run through it would randomly develop too much resistance, an open or a short. Then, either the engine wouldn't start or the PTO wouldn't engage. The tractor was just not dependable, even after throwing new factory switch parts at it. Finally, I just rewired the PTO clutch with a simple heavy duty on-off switch and it's been trouble free for years now.
One "safety" feature it loses is the PTO no longer disengages upon using reverse. The benefit is PTO clutches and belts last much longer. When operating the tractor at normal engine high speed, continually engaging and disengaging the clutch at each reverse is death to the clutch and belts. By having the clutch continually engaged when reversing, the belts and clutch last many times longer. Your opinions and results may vary and this does not constitute a solicitation to bypass any factory systems.
Yes, rewiring to delete the OEM PTO switch makes the mower less idiot proof and no, I don't recommend it for anyone else. It was just the only way to keep using an otherwise serviceable machine. Your opinions and results may vary.
jack vines
